The present invention relates generally to the art of planarizing a workpiece against an abrasive surface. For example, the present invention may be used to planarizing a wafer, or a thin film deposited thereon, in a carrier with adjustable pistons for distributing a pressing force on the back surface of a wafer in a chemical-mechanical planarization (CMP) tool.
A flat disk or xe2x80x9cwaferxe2x80x9d of single crystal silicon is the basic substrate material in the semiconductor industry for the manufacture of integrated circuits. Semiconductor wafers are typically created by growing an elongated cylinder or boule of single crystal silicon and then slicing individual wafers from the cylinder. The slicing causes both faces of the wafer to be extremely rough. In addition, applicant has noticed other semiconductor wafer processing steps, e.g. shallow trench isolation (STI) and copper deposition, produce predictable concentric bulges of excess material on the wafer. For example, applicant has noticed that conventional STI processes usually produce a wide peripheral ring shaped bulge and a small central disk shaped bulge with a narrow trough between bulges. Applicant has also noticed that conventional copper deposition processes usually produce a narrow peripheral ring shaped bulge and a small central disk shaped bulge with a wide trough between bulges.
The front face of the wafer on which integrated circuitry is to be constructed must be extremely flat in order to facilitate reliable semiconductor junctions with subsequent layers of material applied to the wafer. Also, the material layers (deposited thin film layers usually made of metals for conductors or oxides for insulators) applied to the wafer while building interconnects for the integrated circuitry must also be made a uniform thickness. Planarization is the process of removing projections and other imperfections to create a flat planar surface, both locally and globally, and/or the removal of material to create a uniform thickness for a deposited thin film layer on a wafer. Semiconductor wafers are planarized or polished to achieve a smooth, flat finish before performing process steps that create integrated circuitry or interconnects on the wafer. To this end, machines have been developed to provide controlled planarization of both structured and unstructured wafers.
A conventional method of planarizing a wafer will now be discussed. The wafer is secured in a carrier that is connected to a shaft in a CMP tool. The shaft transports the carrier, and thus the wafer, to and from a load or unload station and a position adjacent a polishing pad mounted to a platen. A pressure is exerted on the back surface of the wafer by the carrier in order to press the wafer against the polishing pad, usually in the presence of slurry. The wafer and/or polishing pad are moved in relation to each other via motors connected to the shaft and/or platen.
Numerous carrier designs are known in the art for holding and distributing a pressure on the back surface of the wafer during the planarization process. Conventional carriers commonly have a hard flat pressure plate that is used to press against the back surface of the wafer. As a consequence, the pressure plate applies a substantially uniform global pressure across the entire area back surface of the wafer.
A common problem for conventional carriers having a hard flat plate is that they cannot compensate for incoming wafers that have one or more bulges. The hard flat plate is limited by the fact that it cannot adjust the pressure applied to different zones on the back surface of the wafer. As previously mentioned, it is common for some wafer processing steps to leave bulges on the wafer. Conventional carriers typically remove approximately the same amount of material across the entire front face of the wafer, thereby leaving the bulges on the wafer. Only sufficiently smooth, flat portions of the wafer surface may be effectively used for circuit deposition. Thus, the bulges limit the useful area of the semiconductor wafer.
Other conventional carriers implement means for applying more than one pressure region across the back surface of the wafer. Specifically, some conventional carriers provide a carrier housing with a plurality of concentric internal chambers that may be independently pressurized separated by barriers. By pressurizing the individual chambers in the top plate to different magnitudes, a different pressure distribution can be established across the back surface of the wafer.
However, Applicants have discovered that the pressure distribution across the back surface of the wafer for conventional carriers having adjustable pressure chambers is not sufficiently controllable. This is due to the lack of control of the pressure caused by the barriers on the back surface of the wafer. The barriers are important in controlling the pressure on the back surface of the wafer between internal chambers. Therefore, the ability to control the applied pressure across the entire back surface of the wafer is limited, thereby restricting the ability to compensate for anticipated removal problems. In addition, carriers with chambers that are pressurized with fluids may cause other problems. For example, carriers that use gas to pressurize the internal chambers may inadvertently dry particles on the back surface of the wafer, thereby adhering contaminates to the wafer. Another potential problem exists if a fluid is used to pressurize the internal chambers. The fluid may dilute or contaminate the chemistry being used to planarize the wafer.
What is needed is a system for controlling the application of multiple pressure zones across the back surface of a wafer during planarization to compensate for bulges on the wafer without adhering contaminates to the wafer or diluting the chemistry being used.
Thus, it is an object of the present invention to provide an apparatus and method for controlling the pressure distribution on the back surface of a wafer with independently controllable concentric pistons while planarizing the wafer.
In one embodiment of the present invention, a carrier is disclosed for planarizing the front surface of a wafer. The carrier has a carrier housing that preferably comprises a rigid non-corrosive material. The carrier housing is preferably cylindrically shaped with a first major surface being used to couple the carrier to a CMP tool and a second major surface with a plurality of concentric recesses. A piston diaphragm may be supported by the second face of the carrier housing thereby enclosing the concentric recesses.
A plurality of concentric pistons having a first portion that extends into the concentric recesses in the carrier housing and a second portion for supporting the back surface of a wafer are suspended by the piston diaphragm. The pistons are preferably made as short as possible to maximize the load capabilities and minimize the deflections during the planarization process. The pistons may be keyed to allow for easy assembly of the pistons. The pistons preferably contact mechanical stops on the carrier housing to prevent the pistons from being supported solely by the piston diaphragm when the carrier is lifted off the polishing pad. The portion of the pistons for supporting the backside of a wafer may have a carrier film to assist in the distribution of a pressing force by absorbing minor imperfections on the backside of the wafer.
Apertures may be created in the pistons to communicate a vacuum or rapid discharge of fluid to a back surface of a wafer to assist in holding or releasing a wafer respectively. Alternatively, or in combination, fluid communication paths may be created between concentric pistons to also communicate a vacuum or rapid discharge of fluid to a back surface of a wafer.
A plurality of individually controllable piston fluid communication paths may be in fluid communication with a corresponding plurality of concentric recesses in the carrier housing. The piston fluid communication paths may be used to supply a different pressure on each piston. In general, the more concentric pistons in a carrier, the greater the flexibility the carrier has in controlling the distribution of force on the backside of the wafer. However, every additional piston adds to the complexity and expense of manufacturing the carrier. The radius of the central piston and width of the concentric ring pistons may be varied to assist in optimizing the distribution of force on the backside of the wafer. A carrier having the radius of the central piston approximately equal the ring width of the surrounding pistons avoids the problem of having an extra wide piston. An extra wide piston would prevent adjusting the distribution of force over the area covered by that piston.
Applicant has noticed a common problem of a narrow bulge near the periphery of incoming wafers for certain semiconductor wafer processing steps. The pressure distribution on the backside of these wafers may be more easily optimized by having the ring width of the outermost ring narrower than the ring width of the other rings. The outermost ring may even be narrower than the radius of the central piston.
The carrier preferably has a floating retaining ring connected to the carrier housing. The retaining ring surrounds the wafer during the planarization process to prevent the wafer from escaping laterally beneath the carrier when relative motion is generated between the wafer and the abrasive surface.
A method for practicing the present invention starts by analyzing incoming wafers for a repeating wafer geometry. Some semiconductor wafer processing steps leave predictable concentric bulges on the wafer. The bulges from these processing steps are substantially the same from wafer to wafer in that they often have the same number, position, width and height. By using a carrier with adjustable concentric pressure zones, the carrier may press harder on zones with bulges during the planarization process to produce a wafer with a substantially uniform thickness.
These and other aspects of the present invention are described in full detail in the following description, claims and appended drawings.